Plastic moving-surface treatment of sewage

ABSTRACT

A process for the aerobic treatment of sewage. Raw sewage or the liquid portion of primary settled sewage and the sludge portion of secondary settled sewage are simultaneously aerated in the presence of a plurality of bio-surfaces - a bio-surface being a body capable of attracting and offering surface area for the growth of sewage treating microorganisms and also attracting the nutrients necessary for the growth of these microorganisms and the sewage organic materials to be degraded by the treatment process. The bio-surfaces, therefore, form favorable microenvironments where the microorganisms, their life-sustaining nutrients and organic matter in the sewage are brought together in concentrated fashion which promotes the capacity and rate of sewage treatment. The bio-surfaces comprise synthetic plastic surfaces preferably having a surface charge thereon.

United States Patent Levin 1 Dec. 18, 1973 1 PLASTIC MOVING-SURFACETREATMENT 3293.174 12/1966 Robjohns 1. 210/150 x 0F SEWAGE- PrimaryExaminerMichael Rogers [75] Inventor. Gilbert V. Levin, Chevy Chase, MdAUOmQFD. C Roylance et a! [73] Assignee: Biospherics Incorporated,Rockville,

Md. [57] ABSTRACT 2 Filed: 3 972 A process for the aerobic treatment ofsewage. Raw sewage or the liquid portion of primary settled sewage [21]P 277,504 and the sludge portion of secondary settled sewage are RelatedApplication Data simultaneously aerated in the presence of a plurality[63] Continuation-impart of Ser. No. 2 496 Jan. 13 of bio surfaces abio-Surface being a body capable of 1970 abandoned. attracting andoffering surface area for the growth of sewage treating microorganismsand also attracting 52 us. Cl 210/7 210/11 210/17 the nutrients eeeeseryfer the grewth of these mere" 51 1111. C1. .1 c021: 1/06 erganiems andthe Sewage genie materials to be [58] Field of Search 210/3 9 graded bythe treatment Preeess- The bie'eurfeeee, 21mm 1], 18 15, 17 150, 15]194497 therefore, form favorable microenvironments where themicroorganisms, their life-sustaining nutrients and [56] ReferencesCited organic matter in the sewage are brought together in concentratedfashion which promotes the capacity and UNITED STATES PATENTS rate ofsewage treatment. The bio-surfaces comprise 3,335,081 8/1967 El-Naggar210/15 Synthetic plastic Surfaces preferably having a surface 3,402,1259/1968 Tanaka r 2lO/7 charge thereon 3,236,766 2/1966 'Levm 2l0/63,540,589 11/1970 Boris 210/150 11 Claims, 1 Drawing Figure SEWAGE GRITBIO-SURFACE CHAMBER SOURCE EFFLUENT 5533312 g g 3;: 55$??? BIOTSURFACECHAMBER CHAMBER SEPARATION STATION 1 11 l WASTE RETURN SLUDGE J+ SLUDGEWASTE SLUDGE PLASTIC MOVING-SURFACE TREATMENT OF SEWAGE This applicationis a continuation-in-part of application Ser. No. 2,496, filed Jan. 13,1970 and now abandoned.

Among the sewage treating processes known to the art, two are ofparticular interest. The first is known as the activated sludge process,and the second is known as the trickling filter process.

In the typical activated sludge process, raw or primary settled sewageis mixed with secondary settled sludge to form a mixed liquor which isthen aerated in an aeration basin. In the aeration basin, themicroorganisms graze" upon the mixed liquor, utilizing the organics andnutrients in the mixed liquor for additional cell production andmetabolic energy. In this manner much of the organic matter present ismetabolically oxidized with the result that considerable quantities ofBOD (biochemical oxygen demand) are removed from the sewage. The mixedliquor then flows from the aeration basin into a secondary settlingchamber and, after a suitable settling period, the supernatant isdischarged as a final effluent and all or part of the settled sludgeorganisms are returned to be mixed with the incoming raw or primarysettled sewage for another cycle. Excess sludge, amounts beyond thatneeded to sustain the process, is wasted.

The activated sludge process removes approximately 80 percent to 95percent of the BOD contained in the raw sewage. The activated sludgeprocess requires four to eight hours to achieve its maximumeffectiveness. However, because of increased levels of population, thisdegree of sewage treatment is no longer adequate in many urban areas.

In theory, considerable microbial mass is produced during the aerationof mixed liquor in the activated sludge process. However, in practice,only modest cell population increases occur. In many areas, this modestcell population increase can be explained by the fact that the processoperates at other than the optimum temperature. More importantly,however, a second factor is thought to play a larger role in the rate ofcell population growth. This factor is the concentration of substrateswhich are present in sewage. The concentration of carbohydrates, forexample, is generally two orders of magnitude below that which isrecommended for the culturing of many microbial species present insewage.

The second sewage treating process noted above, namely the tricklingfilter process, relates to a process in which raw or primary settledsewage is permitted to flow through a filter column of broken rock orsynthetic material covered with films of microorganisms. This process,while resulting in only 60 percent to 75 percent BOD removal, does so ina matter of minutes. Thus, the initial rate of BOD reduction in thetrickling filter process greatly exceeds that of the activated sludgeprocess, but does so at the expense of a reduced final level of BODremoval.

The activated sludge process is in greater vogue today than is thetrickling filter process simply because it produces a higher degree ofBOD removal.

As noted above, the initial rate of BOD reduction in the tricklingfilter process exceeds that of the activated sludge process. Since thesewage temperature would be common to both processes, temperature isruled out as the environmental factor causing the difference. It issuggested, then, that one important factor, at least in part responsiblefor the observed differences in the two processes, is the extensivesurface area available to the microorganisms on the trickling filterrocks.

The fact that even extensive contact in the trickling filter will notproduce BOD reduction levels achieved by the activated sludge processmay, on the other hand, be attributable to the higher available oxygenconcentration present and longer contact time in the activated sludgeprocess.

Another possible factor which might further explain the early rateadvantage of the trickling filter is that the trickling filter rocksmight physically concentrate nutrients by adsorption. The resultingpropinquity of the organ isms and nutrients thus promotes biologicalactivity.

SUMMARY OF THE INVENTION The present invention relates to a highlyefficient sewage treating process. The process combines the bestfeatures and advantages of the activated sludge and the trickling filterprocesses in a highly novel and unique fashion and, in operation,accomplishes very high degrees of BOD removal in relatively shorttreatment times.

For conceptual purposes only the novel process may be thought of as anactivated sludge treatment process containing a recirculating tricklingfilter. By the present invention, there is added to the aeration basinin an activated sludge or aeration type of treatment process a pluralityof what shall be termed bio-surfaces" a bio-surface being a particle ofmaterial capable of attracting both the sewage treating microorganisms,the nutrients and organics necessary for the growth of thesemicroorganisms. The bio-surfaces comprise synthetic plastic surfaces.Preferably these bio-surfaces are particles having a specific gravity offrom about 0.9 to about 1.3.

Initially, a charge of bio-surfaces is added, as a slurry, to the mixedliquor at the entrance to the aeration basin. During the aeration stage,the bio-surfaces provide suitable surface area for the growth ofmicroorganisms. Simultaneously, the bio-surfaces concentrate nutrientsand organic substrates through their adsorption mechanisms. Suchconcentration on the bio-surfaces should follow Freudlichs adsorptionisotherm where x is the amount of substance adsorbed, m is the mass ofthe adsorbient, C is the concentration of the substrate and K and a areconstants. In this manner, a myriad of favorable ecological niches ormicroenvironments, is established.

The present invention is based on the theory that if the microorganismsintercept the nutrients and substrates prior to physical adsorption bythe bio-surfaces, or if they successfully remove the substances from theadsorption sites on the bio-surfaces, their metabolism and growth aregreatly enhanced. Microorganisms and the substances they require forsurvival and growth are mutually concentrated on the bio-surfaces.

The bio-surfaces which are added according to the practice of thisinvention are synthetic plastic surfaces such as particles of asynthetic resin formed by either addition or condensationpolymerization. Such resins include thermoplastic polymers such aspolymers of olefins (e.g., homopolymers and copolymers of ethylene,propylene, 1+ butene, etc.) including copolymers with polar monomers(e.g., ethylene-vinyl acetate copolymers); acrylic resins or elastomers(e.g., homopolymers and copolymers with each other or with othermonomers of methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, acrylic acid andmethacrylic acid); polystyrene; copolymers of styrene and other vinylmonomers such as acrylonitrile; vinyl polymers such as homopolymers andcopolymers of vinyl acetate, vinyl chloride, alkyl vinyl ether and vinylbutyral; homopolymers and copolymers of dienes such as polybutadiene,polyisoprene, polychloroprene, butadiene-sytrene copolymers,isobutyleneisoprene copolymers and other unvulcanized elastomersincluding natural rubber. Chemical derivatives of such polymers andcopolymers of ethylenically unsaturated monomers or dienes (e.g.,chlorinated polyethylene, chlorinated polypropylene, chlorinatedisoprene copolymers, salts of ethylenemaleic anhydride, ethylene-acrylicacid, ethylenemethacrylic acid, styrene-maleic anhydride andisoprene-maleic anhydride copolymers, and completely or partiallyhydrolyzed ethylenevinyl acetate copolymers) may also be used. Polymersof cyclic monomers may also be used (e.g., homopolymers and copolymersof ethylene oxide, propylene oxide, tetrahydrofuran, propiolactone,caprolactone, caprolactam and ethylene imine). Copolymers of carbonmonoxide and sulfur dioxide may also be used (e.g., copolymers of carbonmonoxide with ethylene and ethylene imine and copolymers of sulfurdioxide with a-olefins, styrene, vinyl chloride and butadiene).Homopolymers and copolymers of carbonyl compounds may also be used(e.g., homopolymers and copolymers of formaldehyde, acetaldehyde,butyraldehyde, chloral, etc.). Condensation polymers may also be usedsuch as polyesters and alkyd resins which are obtained by thecondensation of a polyhydric alcohol and a polycarboxylic acid. Exampleof polycarboxylic acids which may be used to form the polyester resininclude phthalic acid, phthalic anhydride, succinic acid, adipic acid,isophthalic acid, terephthalic acid, maleic anhydride, etc. Thepolyhydric alcohols which may be used in the preparation of thepolyester or alkyd resin may include the glycols, such as ethyleneglycol, propylene glycol, etc. Polyamide resins may also be used (e.g.,polymers obtained by the condensation of a polycarboxylic acid such asadipic acid, terephthalic acid and dimer acid with a polyamine such asethylene diamine, hexamethylene diamine and diethylene triamine).Polyurethanes may also be used (e.g., polymers obtained by reaction of adiisocyanate such as toluene diisocyanate and 4,4'-diphenylmethanediisocyanate with a polyol such as polyethylene oxide, polypropyleneoxide, polytetramethylene glycol, hydroxyl-terminated polyesters,hydroxyl-terminated polyisobutylene and hydroxyl-terminatedpolybutadiene). Other thermoplastic resins based on bisphenol such aspolycarbonates, polysulfones and polysulfonates as well aspoly-2,5-dimethylphenylene oxide may also be used. Epoxy resins includethe condensation products of bisphenol and epichlorohydrin, epoxidizeddrying oils, the glycidyl ethers of glyceroLepoxylated novolac resins,etc.

The synthetic plastic bio-surfaces may be inorganic particles which arecoated with and coupled to a synthetic resin. U. S. Pat. No. 3,645,939discloses a method for coating inorganic particles such as clay, sandand glass, with a thermoplastic polymer, whereby the polymer is coupledto the thermoplastic polymer. Such particles may be used in the practiceof this inventron.

In the preferred embodiment of this invention, the plastic surfaces areprovided with surface charges. These charges may be positive, negativeor amphoteric i.e., containing both positive and negative'charges. Manybacterial surfaces as well as nutrients, are negatively charged.Therefore, if the plastic surfaces are positively charged, the organismsand the nutrients will be attracted toward the plastic particles at aneven greater rate than when using uncharged plastic particles. Thisresults in an increased concentration of the organisms and thenutrients. The organic substrates, generally bearing little or no chargeare physically adsorbed onto or concentrated near the particles.

Negative charge bearing plastics may be synthesized by the introductionof carboxyl or sulfonic acid groups into the polymer. Dowex 50 is apolymeric material obtained by sulfonation of a copolymer consisting of92 percent styrene and 8 percent divinylbenzene, see W.C. Bauman and J.Eichhorn, J. Am. Chem. Soc. 69, 2830, 1947. A detailed study of thepreparation of sulfonated polystyrene-divinylbenzene resins is describedby K.W. Pepper, J. Applied Chem. 1, 124, 1951. Sulfonated styrene may begrafted to polyethylene using high energy radiation to form an exchangeresin bearing negative charges, see A. Charlesby, Atomic Radiation andPolymers, p. 403, Pergamon Press, N.Y., 1960. Examples of carboxyl groupcontaining polmers are styrene-maleic anhydride copolymers andpolyacrylic acid, see A. Cranshaw and GB. Butler, J. Am. Chem. Soc. 80,5464, 1958.

Synthetic polymers bearing positive charges may be produced by theintroduction of primary, secondary, tertiary, or quarternary aminegroups. Wheaton and Bauman in Ind. Eng. Chem. 43, 1088, 1951, describethe preparation of Dowex l and Dowex 2 which contain positive chargesfrom polystyrene-divinylbenzene copolymers. Positive charge bearingpolymers may also be synthesized by introducing quarternary amine groupsinto a polyethylene-styrene graft copolymer, formed via ionizingradiation, and by quarternization of a vinyl pyridine-polyethyleneradiation induced graft, see A. Charlesby, Atomic Radiation andPolymers, p. 403, Pergamon Press, N. Y., 1960.

Examples of high molecular weight polmeric polymers bearing bothpositive and negative charges (amphoteric polymers) include thosecontaining repeating segments having the structures see Dow ChemicalC0., Midland, Michigan, Tech. Service Bull. 164-62, Ion Retardation, andM. J. Hatch, J. A. Dillon, H. B. Smith, Ind. Eng. Chem. 49, 1812, 1957.

The size of the plastic added surfaces may vary widely and preferably iswithin the range of from about 0.05 to 1.0 mm in diameter. The size ofthe bio-surfaces employed may be mixed or segregated. It is alsocontemplated that the bio-surfaces be of an appropriate specific gravityor have other characteristics such as internal buoyance or the abilityto hold attached gas bubbles, and thus permitting fairly uniform mixingin the aeration basin under the mixing influence of either air or pureoxygen. The quantity of the bio-surfaces may be varied to obtain thedesired surface area-to-sewage volume ratio. The surfaces of the objectsmay be textured or smooth depending upon the desired nature and thelevel of biota to be accumulated thereon. The inert cores may becompletely or partially covered by the active bio-surfaces. Also, thespecific gravity or size of the bio-surfaces may be used as a criteriato assure convenient separation from the mixed liquor and to facilitatethe return of these bio-surfaces to the aeration chamber. Separation maybe by settling or flotation in the secondary settling basin.

The amount of plastic added surfaces used in the practice of thisinvention may also vary widely. The amount used may depend upon thesolids content, the organic content, the phosphate content, etc., of theraw sewage. Generally, from about 1 to pounds of plastic added surfacesper 1,000 gallons of raw sewage, or mixed liquor when sludge isrecycled, is satisfactory. During the aeration process, the bio-surfacescontact the sewage much in the manner of the contact characteristic ofthe trickling filter process. The aeration process, at the same time,highly aerates the liquid medium to promote microbial metabloism for therapid and highly effective treatment of sewage. The plastic surfaces,particularly those with positive charges, attract the microorganismspresent in the mixed liquor as well as the nutrients, principallyphosphates and nitrates, and make them readily available for themicroorganisms. The plastic surfaces also adsorb the organic materialcomprising the BOD load in the sewage, thereby creatingmicro-environments around each surface. This results in a concentrationof the organics and the nutrients at the surfaces and serves to supportthe growth of the microorganisms. Thus, the sewage treatment process isaccelerated by bringing together all of the essential components intoclose proximity at much higher concentrations than occur in a normalsewage treatment process. Accordingly, the practice of this inventionattains a high degree of BOD removal in a relatively short period oftime.

After flowing through the aeration basin for the es tablished detentionperiod, the mixed liquor is discharged into a secondary settling tank.The specific gravity of the bio-surfaces is then advantageously used toimprove the separation characteristics of the sludge inasmuchas thesludge would largely adhere to the biosurfaces. One additional advantageof rapid separation of secondary sludge is the prevention of phosphatedischarge from treated sewage. The separated sludge, in

appropriate portions, is then wasted or returned to the process.

The process of this invention may also be used in conjunction with aphosphate stripping step. Thus, the sludge which is separated from thesecondary settling tank containing a substantial portion of thephosphate orginally present in the influent sewage may be passed to aphosphate stripping zone and treated to cause the microorganisms in thesludge to release phosphate. Such a treatment may be accomplished byholding the mixture under anaerobic conditions as described in U. S.Pat. No. 3,236,766; by aerating the mixture as described in U. S. Pat.No. 3,654,146; or by appropriate pH adjustment. This treatment causesthe organisms in the sludge to release the phosphate which they havetaken up in the aeration tank. The phosphate leaks out of the sludgeinto the liquid phase. A phosphateenriched supernatant liquor isproduced upon settling of the sludge. The sludge is recycled for mixingwith raw sewage which is being fed to the aeration tank and thephosphate-enriched supernatant liquor may be treated to remove thesoluble phosphate therefrom, such as by the addition of a phosphateprecipitant.

Since the sewage treating process of the present invention issubstantially more efficient than those processes known to the priorart, the sewage-handling capacity of presently existing sewage treatingplants may be increased by converting to the process contemplated in thepresent invention and new sewage treating plants may be made morecompact and still have the sewage-handling capacity exhibited by largersewage treating plants employing prior art methods of treatment.

It is evident that by increasing the efficiency and level of BODremoval, the effects on water pollution of the discharge of sewage intoour streams are minimized.

The sole FIGURE is a flow diagram representing the sewage treatingprocess contemplated by the instant invention.

With reference to the FIGURE, raw sewage is introduced into the systemat an optional grit chamber 1. After heavy easily settled materials,such as sand, grit and the like, are removed from the raw sewage by thegrit chamber 1, the sewage to be treated may optionally be fed to aprimary settling chamber 2.

After a time necessary to accomplish the primary settling operation ifsuch is elected, the primary settled sewage flows to an aeration chamber3. However, before, immediately after, or at various points where theprimary settled sewage reaches the aeration chamber 3, it is mixed withan initial charge of plastic bio-surfaces supplied from a plasticbio-surface source 4. Therefore, in the aeration chamber 3, primarysettled sewage and recycled sludge from the secondary settling tank areaerated in the presence of the bio-surfaces. In the aeration tank, themixed liquor is aerated at a rate sufficient to maintain it aerobici.e., so that there is a measurable amount of dissolved oxygen presentin the mixed liquor in at least a part of the aeration tank for a periodof 1 to 8 hours. During aeration, the bacteria present take up phosphateand consume organic matter present in the sewage. A high degree of BODremoval is obtained during aeration.

From the aeration chamber, sewage and the mixed liquor, including theplastic bio-surfaces, are fed to a secondary settling chamber 5 at acontrolled rate providing the desired retention time in the secondarysettling chamber 5. After a time, as determined by chamber design,sufficient for the completion of the secondary settling operation, theclarified liquid flows from the system as an effluent, with or withoutan optional chlorination process.

However, in the event some plastic bio-surfaces remain suspended in theeffluent, the latter may be passed through a bio-surface separationstation 6. At station 6, the plastic bio-surfaces are settled or floatedor otherwise removed and returned to the aeration chamber 3 with orwithout additional treatment.

Also emergent from the secondary settling chamber 5 is the secondaryseparated sludge. This sludge is returned to the aeration basin 3, ormay, alternatively, be expelled from the system as waste sludge, or asin most cases, appropriately divided between the two streams.

It should here be noted that the function of the plastic bio-surfacesource 4 is twofold. First, it serves to inject into the system aninitial charge of plastic biosurfaces. Second, it serves the purpose ofperiodically or continuously adding a make-up quantity of plasticbio-surface to maintain the level thereof at a predetermined and desiredlevel.

The following examples illustrate specific embodiments of thisinvention.

EXAMPLE 1 Raw sewage (one million gallons per day gpd) of approximately150 parts per million (ppm) of BOD and containing 100 ppm of solids ismixed with recyle activated sludge and particles of a synthetic plasticcontaining a positive charge on the surface thereof. The presence of theplastic surfaces results in a higher rate of BOD removal than isobtained when the plastic surfaces are omitted. The effluent mixedliquor from the aeration zone is fed to a secondary settling tank.Clarified waste liquid is discharged to the effluent outflow. Thesettled mixture of sludge and plastic surfaces is withdrawn from thesecondary settling tank at a rate of 150,000 gpd and returned to theaeration basin. Mixed liquor suspended solids are maintained at 2,500ppm. Any excess solids is wasted from the return sludge line from thesecondary settler. The process is operated to maintain a level of about1,000 ppm of plastic surfaces as part of the 2,500 ppm suspended solidsin the aeration zone, based on the amount of mixed liquor in theaeration zone.

EXAMPLE 2 The process of Example 1 is repeated except that the mixtureof sludge and plastic surfaces withdrawn from the secondary settlingtank is passed to a phosphatestripping zone wherein it is held underanaerobic conditions for several hours. The conditions existing in thestripper induce considerable quantities of intracellular phosphate toleak out into the liquid phase. The mixture of phosphate-depleted sludgeand plastic surfaces is recycled and mixed with incoming raw sewage toform the mixed liquor which is passed to the aeration tank. Thephosphate-enriched supernatant liquor is withdrawn from the settlingtank and is fed into a chemical precipitation tank where lime is addedand mixed to form a phosphate precipitate. The phosphate precipitate iswasted and the phosphate-depleted effluent is discharged from thesystem. This process achieves a high degree of BOD removal and a highdegree of phosphate removal from the sewage.

I claim:

1. An activated sludge sewage treatment process which comprises mixinginfluent sewage material with activated sludge to provide a mixedliquor, aerating said mixed liquor in the presence ofa quantity ofparticles of synthetic plastic added surface sufficient to attractsewage treating microorganisms and the nutrients and organic matternecessary for the sustenance and growth of these microorganisms,settling the mixed liquor to separate particles of synthetic plasticadded surfaces, a sludge portion and a clarified liquid portion, andrecirculating at least a part of said sludge and said particles ofsynthetic plastic added surfaces for mixing with the influent sewagematerial.

2. The process as defined in claim 1, wherein said added surfacescomprise plastic particles having surface charges thereon.

3. The process as defined in claim 1, wherein said particles have anaverage particle size of from about 0.05 to 1.0 mm. in diameter.

4. The process as defined in claim 2, wherein said particles compriseinorganic particles coated with a synthetic polymer.

5. The process as defined in claim 1, wherein said surfaces areparticles having a specific gravity of from about 0.9 to about 1.3.

6. The process of claim 2 wherein said sludge before recycling is passedto a phosphate-stripping zone wherein it is treated to cause themicroorganisms in the sludge to release phosphate and provide aphosphateenriched supernatant liquor.

7. The process as defined in claim 1, wherein said surfaces arerecirculated with said returned sludge.

8. The process as defined in claim I, wherein said surfaces arerecirculated after being separated from said clarified liquid portion.

9. The process of claim 1, further comprising the step of adding to thesystem a make-up quantity of surfaces to maintain the concentration ofsame within predetermined limits.

10. The process of claim 1 wherein a portion of said sludge is passed towaste and wherein particles of synthetic plastic added surfaces presentin said waste sludge are reclaimed for reuse in said process.

11. The process as defined in claim 2 wherein said particles ofsynthetic plastic added surfaces are present during the entire aeratingstep.

2. The process as defined in claim 1, wherein said added surfacescomprise plastic particles having surface charges thereon.
 3. Theprocess as defined in claim 1, wherein said particles have an averageparticle size of from about 0.05 to 1.0 mm. in diameter.
 4. The processas defined in claim 2, wherein said particles comprise inorganicparticles coated with a synthetic polymer.
 5. The process as defined inclaim 1, wherein said surfaces are particles having a specific gravityof from about 0.9 to about 1.3.
 6. The process of claim 2 wherein saidsludge before recycling is passed to a phosphate-stripping zone whereinit is treated to cause the microorganisms in the sludge to releasephosphate and provide a phosphate-enriched supernatant liquor.
 7. Theprocess as defined in claim 1, wherein said surfaces are recirculatedwith said returned sludge.
 8. The process as defined in claim 1, whereinsaid surfaces are recirculated after being separated from said clarifiedliquid portion.
 9. The process of claim 1, further comprising the stepof adding to the system a make-up quantity of surfaces to maintain theconcentration of same within predetermined limits.
 10. The process ofclaim 1 wherein a portion of said sludge is passed to waste and whereinparticles of synthetic plastic added surfaces present in said wastesludge are reclaimed for reuse in said process.
 11. The process asdefined in claim 2 wherein said particles of synthetic plastic addedsurfaces are present during the entire aerating step.